AGRICULTURAL USE OF INDIGENOUS
PHOSPHATE ROCKS IN SUB-SAHARAN AFRICA
Satoshi Tobita, Satoshi Nakamura,
Monrawee Fukuda, Fujio Nagumo
Japan International Research Center for Agricultural Sciences (JIRCAS)
Ohwashi 1-1, Tsukuba, Ibaraki, 305-8686, Japan�
Satoshi Tobita holds a doctoral degree in Agriculture from Tokyo University
of Agriculture and Technology (JAPAN) and is currently director of the Crop,
Livestock and Environment Division, JIRCAS. His expertise is in the area of
Crop Physiology and Nutrition. He had worked at ICRISAT, India, as a Post-
Doc, followed by JIRCAS Tropical Agriculture Research Front at Ishigaki,
Japan. As a JIRCAS scientist, he has experiences to collaborate with several
institutions in Sub-Sahara Africa.�
Phosphorus (P) is an essential nutrient for crop growth and production. In Sub-Saharan
Africa (SSA), deficit of soil P is one of the most serious constraints to crop yield. This shortfall has
resulted from the high P fixation capacities of highly weathered acidic soils in SSA, spreading
throughout a range of agricultural lands, from upland to lowland fields. To cope with the P deficiency,
however, resource-poor farmers in SSA cannot apply commercial water-soluble P fertilizers because of
their very limited accessibility and affordability.
As its deposits are found in several places in SSA, indigenous phosphate rock (PR) should be
recognized as a cheaper alternative source of P for local agricultural production. Hence, it is important
to develop appropriate methods for effective utilization of local PRs after knowing their chemical and
physical properties. PRs mined in SSA, with lower water-solubility, are generally considered to have
less effectiveness when they are directly applied onto the fields. Even so, it is worth to evaluate the
effectiveness of PRs on agricultural systems which are differed in crops and environmental (soil and
climate) conditions in SSA.
The JIRCAS Team, collaborating with colleagues in West Africa, has shown a positive effect
of the directly-applied PR from Burkina Faso (BPR) on the growth and yield of lowland rice in
ecologies of Equatorial (Ghana) and Savanna Zones (Ghana and Burkina Faso). BPR could also be
utilized as a delayed-release P fertilizer as it demonstrated a significant residual effect in the same
For the sake of increase in solubility of BPR, several processing technologies have been
examined in consideration of farmers and/or community-based feasibility, e.g., low-temperature
calcination during charring and biological acidulation during composting.
Indigenous resource; Infertile soil; Phosphate rock; Phosphorus nutrition; Rice, Sub-saharan Africa;
Chair Matsumoto: Next speaker is Dr. Satoshi Tobita. He holds a doctoral degree in Agriculture from Tokyo
University of Agriculture and Technology and is currently Director of the Crop, Livestock and Environment
Division in JIRCAS. His expertise is in the area of Crop Physiology and Nutrition. He had worked at ICRISAT,
India, as a Post-Doc, followed by JIRCAS Tropical Agriculture Research Front at Ishigaki. As a JIRCAS
scientist, he has experiences to collaborate with several institutions in Sub-Saharan Africa. His presentation
title is “Agricultural Use of Indigenous Phosphate Rocks in Sub-Saharan Africa.” Dr. Tobita, please.
Dr. Satoshi Tobita: Thank you, Chairman, Dr. Matsumoto-san, and sorry for make you alone. Today, I’ll
speak about phosphate rocks in Sub-Saharan Africa.
Previous speakers mentioned about phosphorus crisis, but we have several possible countermeasures, like
recycling and reuse of the released P in the environment; and second, solubilization of unavailable phosphorus
in the soils; and third, retarding of excess P application through optimization of P fertilizer application; and
exploitation of unutilized P resources. But especially in Africa, there’re several big characteristics in this
situation, one is increasing demand of P application on low P soil conditions and second is the price of the
chemical fertilizer is escalating including P fertilizer, need for affordable and applicable phosphorus resources
for smallholders. Phosphate rocks are promising as an indigenous P resource and they shall be evaluated for
effectiveness and affordability.
We take a look at the phosphate rocks in the Sub-Saharan Africa. There’re many, many phosphate rock mines
in the world, but there is more than 70% of rock deposits in the African continent. These are the map of the
deposits in the African continent. Total phosphate rock production is 28.5% in Africa, but the consumption in
Africa is only 2.8%.
This is some pictures in the rock mines in Sub-Saharan Africa. The left one is in Togo that is very big scale
production of rock phosphates. The right one is from Niger, the name is Tahoua, is a small-scale rock mine.
I’ll show some examples of the chemical composition of phosphate rocks in West Africa. There’re several
variation in characteristics, but the total P is around 10% to 15% and its citrate solubility is around 1% to 2%.
Now we started studies about rock phosphates with use of Kodjari, Burkina Faso rock phosphates. So, this is
about this rock phosphate. It’s 80 million tons of reserve, but this phosphate rocks have relatively low solubility
and high transportation cost so that’s why currently this rock phosphate is only utilized for upland plantations
as a delayed release fertilizer. So, we’ll have some questions and challenges. How to extend the utilization of
rock phosphates for local smallholder farmers? How to increase the solubility of rock phosphates? How about
the lowland conditions?
This is the objectives of this study. The first is to elucidate the effect of direct application of Burkina Faso
phosphate rock on lowland rice yield and secondly is to evaluate the residual effect of Burkina Faso rock
phosphate on subsequent rice yield. So these two objectives are studied in the on-farm experiments in two
lowland ecologies in Ghana and Burkina Faso. Thirdly, we seek the possibility to increase the solubility of rock
phosphate by affordable means of local farmers. So, this experiment is implemented in the laboratory at
glasshouse level in Japan.
First, I’ll talk about the results of the effects of BPR in the on-farm trials in Ghana. In Ghana, there’re two
ecological zones: one is Guinea Savanna and another one is Equatorial Forest. So each ecological zone has the
corresponding rice systems like rainfed lowlands and irrigated lowlands respectively. So, we did experiments
in the two regions and the treatments is one is control and phosphate rock applications with low, medium and
high rates as well as TSP that is water-soluble chemical fertilizer. So, this is the result in terms of the rice grain
yield. In the Guinea Savanna zone that is rainfed lowland ecologies, so rice yield responded very well to the
rock phosphate application in the two sites. Also, in the Equatorial Forest zone, this irrigated rice ecologies, it
also have a good response to rock phosphate application. So that means that rice yield responded proportionally
with BPR application rates in both ecological zones.
This is about the on-farm trials in Burkina Faso. In Burkina Faso, we have eight villages, eight sites for the on-
farm trials. And this is the result. So only the application of rock phosphate has no effect on the rice yield as
compared with zero application. But if we put nitrogen and potassium fertilizer in the optimum levels, so rice
yield responded to rock phosphate, as in this graph, and also responded to TSP. And this tendency was also
observed in the low level of nitrogen application. So, the BPR direct application effectively increased lowland
rice yield in Ghana and Burkina Faso. And rock phosphate application yielded more than 70% of TSP that’s a
chemical fertilizer application. And even in the nitrogen-limiting condition, phosphate rock was effective.
Next we will see the residual effects. That is also from the on-farm experiments in Ghana. The previous PR
plots were divided into two. So, one is residual, which is no more application of P in this current season, and
another one is successive plot that is applied to the PR in this current season. The residual effect was evaluated
in the two lowland rice ecosystems.
This is the results in the Savanna zone, rainfed lowland rice. In the one village, so named Fuu, it has a good
residual effect we can observe. So, we should compare the blue bar and the green bar that is corresponding, the
one is residual and the one is successive. But in another village, named Gbrimah, it has there is no residual
effect in ter